Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices

CIRCULAR No. 292 SEPTEMBER 1933

UNITED STATES DEPARTMENT OF AGRICULTURE
WASHINGTON, D.C.

ARTIFICIAL DRYING OF RICE ON THE FARM

By W. D. Smith, senior marketing specialist, J. J. Deffes, and C. H. Bennett,'
junior supervisors in grain inspection, Gram Division, Bureau of Agricultural
Economics, and W. M. Hurst, associate agricultural engineer, and W. H.
Redit, junior agricultural engineer, Division of Mechanical Equipment,
Bureau of Agricultural Engineering.1

CONTENTS

Page

Introduction 1

Milling quality and market value 2

Methods of harvesting in relation to artificial

drying 4

Windrow harvesting 5

Use of combined harvester-thresher 5

E xperiments in artificial drying 7

Commercial rice driers 9

Installation of the drier 10

Page
Commercial rice driers— Continued

Operation of the drier n

Elements of cost 14

Experimental driers 18

Temperature of drying air 19

Air velocity and rate of drying 21

Effect on germination 24

Practical conclusions for the trade 24

INTRODUCTION

Observations made on a farm in the South a few years ago indi-
cated that the combined harvester-thresher might be used to advan-
tage in reducing the cost of harvesting rice, but that it would be
necessary to dry the rice artificially if the combine were used. Since
that time several farmers have used the combine to harvest their
rice crop and have dried the threshed rice with commercial driers.
But observations and tests made on farms in Arkansas and in Texas
where combines and artificial driers were used in 1929, 1930, and
1931 show that the regular grain combine is not entirely suitable
for harvesting rice in those areas, both because of the presence of
wet fields and irrigation levees, and of the green or immature condi-
tion of the rice straw at harvest time. Attempts are being made by
some farm-machinery manufacturers to develop a combine that will
be suitable to rice-field conditions.

The investigations here reported show that the method of combine
harvesting and artificial drying, if properly carried out, eliminates
damage to the rice from unfavorable wTeather, reduces loss from
shattering, and produces rice of higher milling quality and a more
uniform product than does the common method of harvesting with
a binder. Tests made with both experimental and commercial grain
driers show that in order to obtain rice of a high milling quality
a much lower drying-air temperature must be used than is customary
in drying other cereals. If a drying-air temperature as high as

1 Acknowledgment is made to John Mc Williams, Jr., and William Lawlor, Jr.
facilities for conducting these investigations.

183189°— 33 1

who made available

2 CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

120° F. is used, the moisture content of rice should not be reduced
more than about 2 percent at any one drying operation, unless that
content is considerably in excess of 20 percent. If sufficient bin
space is not available for storing the rice between drying operations,
or if for other reasons it is necessary to dry the rice in one operation!
the drying-air temperature should not exceed 110° for best results.

If rice is cut at the proper stage of maturity with the binder,
is properly shocked, and no unfavorable weather occurs at harvest
time, the binder method can usually be depended upon to produce
rough rice of good quality. But in the rice-producing areas of the
South there is a loss in the field through shattering in the handling
of the bundles from the time they are dropped from the binder until
they are placed in the threshing machine, and it is difficult to obtain
rough rice of high quality and of good condition if the weather is
unfavorable during the harvest, or during the time the grain is in
the shocks.

Ordinarily, when rice is harvested with binders, the kernels have a
20 to 30 percent moisture content at time of harvesting ; if allowed
to stand in the field before harvesting until the moisture content is
much lower than 20 percent, the kernels frequently become so checked
or cracked that many of them break in milling. As the moisture
content of rough rice should not exceed about 1-4 percent for safe
storage and for high milling quality, binder-cut rice must remain in
shocks to dry before being threshed. If rains are frequent or if the
atmospheric humidity is high during this time, it is often almost
impossible to obtain dry rough rice. Damp rice is difficult to
thresh out of the heads, and in such cases some rice is usually lost
with the straw.

MILLING QUALITY AND MARKET VALUE

The market value of a given lot of rough rice is determined largely
by the percentage of whole-grain milled rice that can be obtained
from it. Kough rice may be dry and comparatively free of the
visible defects that affect its market value and yet be of relatively
low milling quality because a high percentage of broken milled rice
is likely to result from these defects. The hardness of rough rice
(its resistance to breakage in milling) is generally referred to as
" milling quality ", therefore it is frequently said that milling quality
chiefly determines the market value of rough rice.

Table 1 shows how the market value of rough rice would be in-
creased with each pound of increase in yield of head rice when the
milled rice 2 is valued at 2V2 cents per pound for head rice and 1 cent
per pound for broken rice 3. If the yield of total milled rice remains
the same in all cases, an increase of 1 pound in the quantity of head
rice results in an increase in price of iy2 cents per barrel of rough
rice. On this basis the milled rice obtained from a barrel of rough
rice yielding 80 pounds of head rice and 110 pounds total milled
rice has a value of $2.30, whereas the milled rice obtained from a
barrel of rough rice yielding 90 pounds of head rice and 110 pounds

2 Rough rice is rice from which the hulls have not been removed ; head rice is milled
rice consisting principally of whole kernels ; and milled rice is rice from which the hulls,
germs, and practically all of the bran layers have been removed.

3 All estimates for yield of head rice in table 1 are based on tests made with the Smith
shelling device.

ARTIFICIAL DRYING OF RICE ON THE FARM

total milled rice has a value of $2.45. The difference is 15 cents per
barrel of rough rice. (A barrel of rough rice is 162 pounds.)

Table 1. — Value of milled rice obtained from one barrel of rough rice when
total yield remains the same (110 pounds) and head rice yield is increased,
calculated on price basis of 2V-2 cents per pound for head rice and 1 cent per
pound for broken rice.

Yield of
head rice
per bar-
rel

Yield of
broken
rice per
barrel

Value of
head rice

at2J4
cents per

pound

Value of
broken
rice at

1 cent per
pound

Value of
milled rice
obtained
from one
barrel of
rough rice

Pounds

Pounds

Dollars

Dollar

Dollars

75

35

1.87*4

0.35

2.22J4

76

34

1.90

.34

2.24

77

33

1.92J4

.33

2.25^

78

32

1.95

.32

2.27

79

31

1.97J4

.31

2.2834

80

30

2.00

.30

2.30

81

29

2.0234

.29

2.3134

82

28

2.05

.28

2.33

83

27

2.073^

.27

2.3434

84

26

2.10

.26

2.36

85

25

2.12H

.25

2.3714

86

24

2.15

.24

2.39

87

23

2.1734

.23

2.4034

88

22

2.20

.22

2.42

89

21

2.22^

.21

2.43H

90

20

2.25

.20

2.45

91

19

2.2734

.19

2.4634

92

18

2.30

.18

2.48

93

17

2.32J4

.17

2.4934

94

16

2.35

.16

2.51

95

15

2. 371-6

.15

2.5234

96

14

2.40

.14

2.54

97

13

2.42J4

.13

2.5534

98

12

2.45

.12

2.57

99

11

2.47H

.11

2.5834

100

10

2.50

.10

2.60

The effect of the loss of moisture on the hardness of rough rice is
indicated by results obtained in experiments in which wet rough rice
was allowed to dry naturally. Samples of wet lots of rough rice
were placed in open pans in a laboratory. The samples were tested
daily for moisture and for milling quality. Figure 1 shows that
as the rice became drier there was in each case a corresponding
improvement in the milling quality of the rice.

When damp or wet rice is milled a large number of the kernels
break, thus reducing the yield of head rice per barrel of rough rice.
The yield of head rice usually increases with a decrease in moisture
until the moisture content has been reduced to between 12 and 14
percent.

Other things being equal, dry rough rice is of higher milling
quality than is rice with a high moisture content, and it is very
important, from the standpoint of market value, that the moisture
content of all rough rice be reduced to the point at which the rice
attains its maximum milling quality. Since there is a very consid-
erable difference in value between similar lots of dry rough rice of
high and of low milling quality, it is very important that the
artificial drying of the rice be performed by a method that will
harden the rice and render it resistant to breakage in milling.

Each season the moisture content of rough rice has a material
effect upon the marketing of the milled rice produced from the crop.

4 CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

TThen the crop is damp or wet a large part of the rice heats or be-
comes stack-burned while in storage, and the resulting milled rice
is of poor appearance. Such milled rice is difficult to sell. Milled
rice containing excess moisture sometimes heats and becomes spoiled
while in transit ; this causes disputes between buyers and sellers. As
foreign buyers object to milled rice that contains more than 14.5
percent moisture, exports of milled rice almost invariably decline
during wet seasons owing to excess moisture in the milled product.
The moisture content of rough rice produced in the Southern
States during the 5 years 1922 to 1927 inclusive, is shown in figure 2.
According to the 3,300 samples tested, the average moisture content
for each crop of rice, by years, ranged from 12.6 to 14.8 percent.
The proportion of the crops that contained excess moisture (more
than 14 percent) varied from year to year. According to the sam-
ples tested only 18.6 percent of the 1921 crop contained excess mois-
ture, though the percentage for the 1925 crop was slightly more

Moisture (percent)

Figuee 1. — Increase in estimated yield of head rice with loss of moisture in rough rice
when dried under normal atmospheric conditions.

than 69. Only about 51 percent of the samples contained 14 percent
or less of moisture. About 29 percent of the samples contained more
than 14 but not more than 15.5 percent of moisture, and were classed
as damp : about 15 percent of the samples contained more than 15.5
but not more than IT percent of moisture, and were classed as wet;
and about 4.5 percent of the samples contained more than IT percent
of moisture and were classed as Sample grade.

METHODS OF HARVESTING IN RELATION TO ARTIFICIAL DRYING

During seasons when prices are low it is particularly important
to the grower that he obtain as high a price as possible. The cost of
producing low-quality and high-quality rice is roughly the same.
There are certain fixed charges that do not vary a great deal with
the quality of rice produced. The profit or loss on the farming oper-
ations for the year may be determined to a large extent in the
harvesting. The milling quality of the rough rice and its market
value are governed largely by weather conditions and the method of

ARTIFICIAL DRYING OF RICE ON" THE FARM 5

harvesting. Some of the newer harvesting methods that have been
tried in the Southern States, in an effort to reduce harvesting costs
and to obtain high-quality rice during wet seasons, have employed
the combine-harvester, windrower, and the header.

WINDROW HARVESTING

The windrow method of harvesting in the South is handicapped
by weather hazards. Heavy rains frequently flood the rice fields
during the harvest season to a depth of several inches. Even under
ordinary conditions the milling quality of rough rice may be im-
paired by the repeated wetting and drying which result from dews,
rains, and sunlight. The milling quality of windrowed rice may
suffer considerably if the moisture content is reduced too rapidly.
Likewise, if the heads hang downward or if they are exposed on top

1922 23

1923-24

1924-25

1925-26

1926-27

DRY (UNQUALIFIED

GRADES)

DAMP

WET ,

VERY
WET|

GRADE

1 1 1 1

>'■

1

\ SAMPLE

\ 1

1 1

1

P""~-

--!-

^»

J

1 1

J_.

-A--

— 1"

7->-r

/ 1

i i iumTiim

10

20

m

Moisture /4 percent or /ess;
no penalty in grading

40 50 60 70

PERCENT OP SAMPLES
Moisture content yao« Moisture content
l<t.l to IS. 5 percent KmS 15.6 to 17 percent

80

90

100

Moisture content
17.1 percent and over

FiGrRB 2. — Moisture content of rough rice produced in the Southern States, 1922-27,
showing the variation in the percentage of samples of rough rice that contained 14
percent or less of moisture, and that graded as Damp, Wet, and Sample grade, on the
moisture factor, during the years 1922-27.

of the swath the rice kernels are likely to be damaged badly in wet
weather.

To determine whether a windrower equipped with an elevator for
loading the headed rice into a wagon pulled beside the windrower
could be used advantageously when a drier was available, such equip-
ment was tried in Arkansas during the 1930 harvest season. The
light windrower can be operated in wet fields, but some difficulty was
experienced in keeping the wagon under the elevator when crossing
levees. Difficulty also was experienced in threshing the headed rice
with a stationary thresher. Therefore attempts to use the machine
were not made on other farms.

USE OF COMBINED HARVESTER-THRESHER

The combined harvester-thresher, generally known as the com-
bine, cuts and threshes at one operation. It was developed prima-
rily for harvesting wheat, but it has been used with considerable
success for harvesting a variety of similar crops.

6

CIRCULAR 2 9 2, U.S. DEPARTMENT OE AGRICULTURE

In the fall of 1929, observations and tests were made on one farm
in Texas and another farm in Arkansas, on which rice was harvested
with a combine and dried with a commercial drier. Further obser-
vations and tests were made on the same farms during the harvest
seasons of 1930 and 1931. Field observations and studies dealing
with some of the mechanical and economic factors involved in
artificial drying were also made.

On the farms under observation it has not been possible to harvest
as many acres per day with a combine as with a binder with the
same width of cut. Apparently a rice grower may have to operate
as many combines in a field as he has been operating binders in
order to cut all the rice at the proper stage of maturity, at least
unless mechanical improvements are made in the combine that will
render this machine better suited to rice-field conditions. To
attempt to harvest a large acreage of rice with only one combine

Figure 3. — A 12-foot combine pulled by two tractors.

would probably mean that some of the rice would become overripe
and be of poor milling quality before being cut.

Low wet spots and weed patches, and high narrow levees that
run through the fields make it difficult to harvest rice with a com-
bine. Bice growers who have operated combines realize that their
levees must be made low and broad. Figure 3 shows a 12-foot com-
bine in operation in a rice field in which it was necessary to use two
tractors.

In the South the rice stalk, or stem, is usually green at harvest
time; the warm humid weather that frequently prevails during the
harvest season and the water-soaked soil often combine to keep the
plants alive after the rice is ready to harvest. The combine can
cut and thresh rice under such conditions but the green straw has
a tendency to clog the machine, often necessitating frequent stops.

There are numerous advantages, however, in harvesting rice with
a combine and drying it artificially. In one operation the rice is
removed from the fields with no danger of weather damage ; there is
very little loss of rice through shattering; rice of high milling

ARTIFICIAL DRYING OF RICE ON THE FARM 7

quality can be obtained regardless of weather conditions at harvest ;
and all of the rice can be thoroughly and uniformly dried, making
it safe for storage either in bulk or in sacks. Combine-harvested
rice that has been artificially dried is usually more uniform in
quality than rice handled in the customary way; this is because of
the thorough mixing of the lot during the drying operations.

EXPERIMENTS IN ARTIFICIAL DRYING

When rough rice is properly dried artificially usually the hardness
of the kernels increases as the moisture content of the rice decreases.
As the principal objective is to lower the moisture content so as
to increase the hardness of the rice, the temperature of the drying
air must be high enough to remove the moisture from the rice, but
not so high that it will injure the milling quality.

In the artificial drying of rough rice observed in Arkansas, drying-
air temperatures of from 94° to 113° F. were used; the temperature
usually averaged between 100° and 110° with an exposure of
approximately 1 hour for individual operations.

In the operations observed in Texas, a drying-air temperature of
approximately 120° F. was used. It was found that, by increasing
the temperature to 120° and reducing the period of exposure, the
capacity of the drier could be increased materially without injuring
the quality of the rice. But for the best results when a drying-air
temperature as high as 120° is used, the moisture content should not
be reduced more than 2 percent further at any one drying operation
after it has reached 20 percent.

Table 2,

-Hardening effect on kernels when damp or wet rough rice of several
varieties was dried artificiallu

Moisture

Whole kernels in shelling test

Indi-

cated

yield of

Variety

head rice

Before
drying

After
drying

Loss

Before

drying

After
drying

Increase

per barrel
after dry-
ing

Percent

Percent

Percent

Percent

Percent

Percent

Pounds

{ 19.9

14.3

5.6

54.6

63.8

9.2

86

Storm Proof

18.3
17.1

14.9
13.4

3.4
3.7

53.0
60.5

66.3
71.9

13.3
11.4

89

97

20.7

14.9

5.8

45.9

67.9

22.0

91

17.5

13.7

3.8

72.8

74.8

2.0

101

22.2

14.5

7.7

42.5

62.3

19.8

84

17.0

14.4

2.6

61.1

66.0

4.9

89

22.9

14.7

8.2

36.7

65.8

29.1

89

23.6

14.8

8.8

38.0

66.9

28.9

90

15.3

14.7

.6

66.3

68.6

2.3

92

23.6

14.1

9.5

37.8

67.9

30.1

91

26.8

14.1

12.7

25.1

69.3

44.2

93

24.5

14.0

10.5

45.0

68.2

23.2

91

24.1

14.3

9.8

40.4

69.8

29.4

94

24.1

13.9

10.2

39.1

68.0

28.9

91

Early Prolific

26.5

14.5

12.0
8.1

29.9
41.6

69.1
63.8

39.2
22.2

93
86

22.6

14.5

22.5

13.4

9.1

47.4

67.3

19.9

91

21.0

13.6

7.4

54.2

67.3

13.1

91

14.9

14.0

.9

61.1

65.6

4.5

88

15.6

14.6

1.0

64.7

65.8

1.1

89

16.5

14.3

2.2

66.3

68.8

2.5

93

16.3

14.9

1.4

60.7

63.8

3.1

86

15.6

14.5

1.1

64.3

65.2

.9

87

15.8

14.4

1.4

58.1

63.4

5.3

85

17.0

14.7

2.3

53.3

64.6

11.3

87

20.1

14.8

5.3

50.3

59.0

8.7

80

I 20.3

14.4

5.9

43.7

59.3

15.6

80

8 CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

Table 2. — Hardening effect on kernels when damp or wet rough rice of several
varieties was dried artificially — Continued

Moisture

Whole kernels in shelling test

Indi-
cated

Variety

Before
drying

After
drying

Loss

Before
drying

After
drying

Increase

yield of
head rice
per barrel
after dry-
ing

Fortuna

Percent
{ 18.7
J 14.7
1 15.5
I 15.5
f 16.3

24.8
{ 25.1

27.4
I 26.8

19.4

Percent
14.7
13.8
13.7
14.1
14.2
13.8
13.9
14.6
14.1
13.7

Percent

4.0

.9

1.8

1.4

2.1

11.0

11.2

12.8

12.7

5.7

Percent
23.2
39.0
43.4
38.1
73.6
38.0
34.9
29.7
26.8
46.4

Percent
56.8
50.7
55.5
46.4
75.9
75.0
72.3
73.8
74.7
72.0

Percent
33.6
11.7
12.1
8.3
2.3
37.0
37.4
44.1
47.9
25.6

Pounds
78
73

Blue Rose

77
69
102
101

97
99
100

97

The hardening effect on the kernels when damp or wet rough rice
was dried artificially is shown in table 2. In table 2 the results ob-
tained by artificial drying of several lots of different varieties are
shown. For each lot there was a very appreciable increase in hard-
ness, resulting in an increase of whole milled-rice kernels when rough
rice with a high moisture content was dried to a lower moisture
content.

The milling quality of each lot, as indicated by the official shelling
test, is shown 'in table 2. For most of these lots the indicated yield
of head rice is as high or higher than is usually expected from an
average lot of rough rice of the variety involved. The milling
quality of some of the lots probably would have been higher had the
moisture content of these lots been reduced to 14 percent. Reports
obtained of the actual milling outturns from several of the lots dried
under observation show that better-than- average mill yields were ob-
tained from some of the lots.

When samples of the artificially dried rice were tested immediately
after drying, and after the lapse of some time, it was indicated that
such rice improves in milling quality when allowed to remain in
storage for a few days after drying. This is probably because of
the unequal distribution of moisture in the outer and inner parts
of the kernel immediately after drying. When an even diffusion of
moisture has been accomplished in the kernel it is more resistant
to breakage.

Evidently rough rice that is artificially dried on a farm should not
be shipped to a mill for immediate milling, and probably should
not be offered for official grading or for sale immediately after being
dried. A better indication of milling quality will be obtained if the
rice is tested after it has been stored for a few days.

When a drier is installed at a rice mill it apparently would be
desirable to have a bin between the drier and the mill and to hold
the dried rough rice for a time before milling instead of milling it
immediately as is usually done at present.

At one of the driers under observation there was so much rice to be
dried that, to protect the quality and condition of each lot, it became
the practice partly to dry each iot as soon as it was brought from the

ARTIFICIAL DRYING OF RICE ON THE FARM 9

combine. The rice was then put into a bin where its partly dried
condition insured that it would keep in good condition for a short
time until the moisture content could be reduced by further drying.
Usually, a day or two after the first drying operation, 2 or 3 of these
lots were combined and given a second drying. In some instances 2
lots were put together for the second drying operation, a third being
added for a third drying. By this method the moisture content of
all the rice was reduced somewhat immediately after it was harvested.
It then remained temporarily in good condition, elevator bin space
was conserved, and time was saved through drying large lots.

So long as the rice is all of the same variety, type, and quality, it
will be found more economical to handle and dry it in large lots.
Care should be used to prevent mixing varieties in the drying opera-
tions. On many farms on which only one early variety and one late
variety are grown there will not be great danger of mixing, as the
early rice will probably be harvested and dried before the late rice
is ready for harvesting. If more than one early or one late variety
is grown, or if a variety is grown that matures between the early and
late varieties, the drier, the elevator, the conveyor, and the bins
should each be cleaned thoroughly of one variety before another is
handled, because the mixing of two varieties usually results in a
lowering of the market value of both.

Bice cut with a combine should usually be cleaned before it is
dried. The foreign material usually has a high moisture content
and is often of a kind that retards the flow of rice through the drier ;
its removal enables the drying air to be used solely for the rice and
there is a more even flow of the grains. The cleaning machinery
can also be used later for removing the lightweight rice kernels and
small fragments of foreign material that may remain in the dried
rice. This latter cleaning usually improves very materially the
quality and market value of the rice.

COMMERCIAL RICE DRIERS

Few if any of the driers manufactured in the United States are
designed specially for rice. Driers commonly used in the South for
this crop are generally known as grain driers and are suitable for
use in drying practically any cereal grain. Such driers may differ
in details of design and construction but practically all of them are
similar in that the drying air is forced through thin layers of grain —
4 to 6 inches — for uniform drying. Heat may be supplied by steam
coils or directly from a furnace. When steam is used, air is drawn
or forced through a nest of steam coils and then is forced through
the rice by a fan. With the direct-heat type of drier flue gases are
drawn from a furnace, mixed with outside air in a mixing chamber,
and then forced through the rice with a fan (figs. 4 and 5).

Most commercial driers are composed of two compartments, one
above the other. The upper compartment is supplied with heated
air for drying and the lower compartment with atmospheric air for
cooling. The cooler is not so essential for rice as for some other
grains, as much lower temperatures are usually used in drying rice.
The temperature of the rice when discharged from the drying com-
partment is usually from 10° to 20° F. lower than the temperature

183189° — 33 2

10

CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

of the drying air used, depending largely upon the temperature of
the drying air and the moisture content of the rice. The evapora-
tion of moisture from the kernels tends to keep the rice cool.

Practically all driers used for rice are so designed that the rice
can be dried either in batches or by continuous flow through the
drier. In batch drying the rice is held stationary in the drier com-
partment for the desired length of time and then dropped into the
cooler, after which the drying compartment is again filled with
undried rice. In the continuous-flow method, which can be used
only when there is more than enough rice in a lot to fill the drier,
the first charge is dried as in batch drying and then allowed to

remain in the cooler
[• \@ for 15 or 20 minutes

before the discharge
mechanism is started
for the continuous-
flow process.

INSTALLATION OF THE
DRIER

Rice driers are usu-
ally located in or ad-
jacent to an elevator,
warehouse, or rice
mill (figs. 6 and 7).
The drier should be so
located and equipped
with conveying and
cleaning machinery
that it can be oper-
ated without interfer-
ing with other opera-
tions about the plant.
When rough rice is
dried artificially in a
commercial drier the
rice passes through
the drier in bulk. It
is essential for econ-
omy of operation that-
sufficient bin space be
provided to handle the rice in bulk before drying, between drying
operations, and immediately after drying. When rice is adequately
dried it is safe for storage in bulk and it is economical to continue
to handle it in bulk until it is milled, if sufficient storage space is
available.

As damp or wet rice tends to clog in pipes and hopper-bottom
bins, it is necessary to make all pipes and chutes larger and to give
them more slope than is customary when dry rice is handled. Hop-
per bins should also be given more slope when they are to be used
for damp rice.

Considerable time and labor can be saved in operation if ample
provision is made for conveying the rice to and from the drier at a

Figure 4. — Diagrammatic section of rice columns in
drier. Nome. Tex.

ARTIFICIAL DRYING OF RICE ON THE FARM

11

rapid rate. In some cases it has been found desirable to have a bin
so located that the drier can be filled therefrom by gravity. This
arrangement allows the operator to fill the drier quickly and reduces
the time required to shift from one lot to another when the lots must
be kept separate. It may also be possible for the operator to feed
the drier from this bin and to shift rice from one bin to another with
the elevator that is used for supplying rice to the drier.

For conveying the rice from bins to the drier, or from the drier
to the bins, belt conveyors have an advantage over screw conveyors
in that the former clean themselves, thus preventing the mixing of
varieties when lots of different kinds are being handled. It was
observed at one place that the bins were constructed in pairs and
in line, so that only one belt conveyor was necessary for conveying
the rice from any bin to the elevator.

In the installation of a drier and in the design of the building
which houses the drier, provisions should be made for discharging
the exhaust air from the drier and preventing its recirculation
through the rice. If
the air is used in
the drier more than
once it may become
saturated with mois-
ture at a high tem-
perature and retard
the evaporation of
moisture from the
rice. In such cases
the rice may actually
absorb moisture
from the air in-
tended for drying.
It is also important
that air-intake ports
for both the drier
and the cooler be

well protected from the weather. If this precaution is not taken,
rain water may be drawn in with the air; the temperature of the
air is thus raised and the water evaporates; this increases the rela-
tive humidity and decreases the drying capacity of the air. When
moisture-laden air is drawn into the cooler the rice may absorb as
much moisture during the cooling process as was evaporated while
it was in the drier.

/

/.

\

Figure 5.

/

/

/

/

-Diagrammatic section of rice columns in drier,
DeWitt, Ark.

OPERATION OF THE DRIER

It is advisable to have the drier under the supervision of one per-
son who has no other duties to perform. Driers are designed to
operate with little attention but better results are obtained if they
are under constant observation. To dry rice artificially so that the
dried rice shall be of good milling quality, it is necessary to control
the temperature of the drying air, to know the moisture content of the
rice before and after each drying operation, and to be able to expose
the rice to the drying air for any desired length of time.

12

CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

Some driers are equipped with a thermostat by means of which the
drying-air temperature can be controlled automatically, but if the
thermostat fails to function properly, considerable damage may be
done to the rice, if prompt attention is not given.

Figure 6. — Elevator and drier installation on a farm near DeWitt, Ark.

In sampling the rice for moisture determinations, the samples
should be taken so that the results will indicate the average moisture
content of the lot. After the moisture content of the rice has been
ascertained, a decision should be made as to the number of drying
operations required.

FlGUEE

Warehouse and drier installation, Nome, Tex.

Most driers have a mechanical device for discharging the grain
from the drier at any desired rate. The rate of discharge, in most
cases, will depend to a large extent on the moisture content and
physical condition of the grain. Rice will usually be discharged at a
much greater rate with the same setting of the discharge mechanism

ARTIFICIAL DRYING OF RICE ON THE FARM

13

after than during the first drying operation. The quality of foreign
material as well as differences in shape and size of the kernels of dif-
ferent varieties also affect the rate of discharge on some driers. The
operator should check the rate of discharge frequently to be sure that
the rice is being exposed for the proper length of time. If the rice
is exposed too long it may be injured and if for too short a period
an extra drying operation may be required.

A check may be made by noting the time required to fill a container
of given volume or by weighing the rice that comes through in a
given length of time. The period of exposure can be determined if
the capacity of the drying compartment and the number of bushels
the drier is discharging per unit of time are known. For example,
the drying compartment in the drier under observation at Nome,
Tex., held approximately 72 bushels. If it was desired to expose the
rice 30 minutes, the discharge was set so that 144 bushels would be
discharged per hour, or 2.4 bushels per minute.

It is helpful to the operator to know the rate at which the drier
will remove moisture from rice of a given moisture content and

Time (hours)

Figure 8. — Rate-of-drying curve for commercial drier at Nome, Tex. An average drying
air temperature of 123° F. was maintained and the rice was exposed to this air in the
drier for approximately 36 minutes each day, the process being repeated until the
moisture content of the rice had been reduced to about 14 percent.

with air at a given temperature. A curve was plotted representing
the rate of drying at a temperature of approximately 120° F. for
the drier at Nome, Tex., and it was found to be very useful in deter-
mining the time of exposure necessary for removing a given quantity
of moisture. Such a curve is shown in figure 8.

To illustrate the use of this curve, it will be assumed that a lot
of rice with a 24 percent moisture content is to be dried to 14 percent
moisture. (Tests have shown that the rice will not be injured if
the moisture content is reduced as much as 4 percent during the first
drying operation, provided the rice contains moisture much in excess
of 20 percent.) It is decided to dry the rice four times, with a
drying temperature of 120° F., removing 4 percent moisture during
the first drying operation and 2 percent during each of three other
.successive operations. Note the point where the 24 percent moisture-
content line intersects the curve and then the point where the 20
percent line does so. It will be seen that an exposure of approxi-
mately 50 minutes would be required for the drier to reduce the
moisture of the rice from 24 to 20 percent. For the next three drying-
operations an exposure of about 30 minutes each would be required.

14 CIRCULAR

Commercial rice driers will usually clo the work for which they
are designed without changes or alterations on the part of the
operator. In some cases, however, it has been found necessary to
make certain changes. To dry rice satisfactorily the drying air must
be forced through the rice or otherwise brought in contact with each
kernel. Because of the design of certain types of driers or because
of the temperature and pressure of the drying air used, the air
sometimes passes from an intake to an exhaust port without passing
completely through the column of rice. When this occurs it is neces-
sary to close such ports on the intake and exhaust sides of the drier
as to cause the air to pass directly through the columns of rice in
the drier.

The rice grains will " funnel " in some types of driers when rice
is being dried in a continuous flow through the drier. When this
occurs the rice near the center of the drier is exposed to the drying
air for only a few minutes whereas that near the sides of the drier
may be exposed for several hours. In the case of one drier in
which funneling was reported, it was found that angle-iron supports
for the cut-off valves between the drier and cooler, and between the
cooler and the discharge hopper, were placed near the outer edges
of the drier. These supports were shifted to a position near the
center of the drier and placed about 6 inches apart, which changes
practically eliminated funneling. The opening in the bottom of the
discharge hopper extended across the center of the drier and at right
angles to the columns or ribbons of rice in the drier. This had a
tendency to make the rice funnel which was augmented by the
obstruction in the form of valve supports near the sides of the drier.

ELEMENTS OF COST

The power, labor, and fuel requirements in the operation of a
commercial drier depend to some extent on the type and size of the
drier used, skill of the operator, weather conditions, and the quantity
of rice dried in continuous operation. As a general rule the cost of
operating a large drier is less per bushel of rice dried than for a
small drier of the same make, because labor requirements are about
the same regardless of the size of unit, and less power is required
per unit quantity of rice dried. The operator of practically any
drier, however, can reduce the cost of drying by timely firing and by
so arranging the drying operations as to keep the drier in continuous
operation over long "periods. As much fuel may be required to get up
steam or to heat a furnace as is required to dry a small lot of rice.

Table 3 shows the approximate operating cost (power, labor, and
fuel) for individual drying operations for several lots and varieties
of rice, based on tests made during the 1930 harvest season at De-
Witt, Ark., and during the 1931 harvest season at Nome, Tex. The
cost of man labor, fuel, and electric current is based on prevailing
prices on the farms referred to for the 1930 and 1931 harvest seasons.
The actual power, labor, and fuel requirements in drying a given
quantity of rice would probably vary only slightly at these plants
from year to year, but the unit cost of such services has changed
somewhat since 1931.

ARTIFICIAL DRYING OF RICE ON THE FARM

15

CO 1

N

1 I i

^,

c

•1-5

03~

^2

*» «o

t-

>o

r^

8

=2©

3*1

o
o

o

s

'■§

£oo 1

Q?

OfflCO

CM

s

H

03

So

5

«D»I>

o

s

a

o

1*|

CO'* r-i

o

«d

O

E-

CO

TTI

^

£ ■*

o

NOtO

CO

CO

SM

CM

IOCJ3 5C

oa

"3

o

"S "^

•^

1^

CO

CO

3

s

fiM

«o

o

2^S

o

CO

CM

CO

c3 >>

00

oo

CO >o CO

CM

3+2

cy-S

I

Jg

£co 1

CC

<M y-t CO

,_,

,_,

.SI

lO

©NCO

t*

05

d
O

r~

t^ t^ «3

_;

CD

u

<N

o

rQ

C3

03

") § lO

>o »o >o

lO

o

g

o

CO

"C-lTf

ICHCN

CO

CO

+=

£^H 1

M

rtoocr.

00

(-^

_«»

CM

*oo

r^

i

o
O

<3

©

■** -*< CO

<m'

Th

, £

*- 1

o

SJfr-

1^ CD CO

CO

<Si

*

■2 "5
1 1

CO

rt<coM

5

>o

t. §

ol

lO

■ONlO

05

0) .—

° ^

c

r^ ci o

s

r-< ^ a:

1-1

©£

■w «C

o

t^CM CM

o

£o

53 'S

Si

35

>o r^cr>

oS

©

CJ "O

CO

lO 00 lO

CO

<

X)

ft

o

£

£

30

ho> CO

$ 03

fcjoo

CN CO O

im'

<

Is

oc

t> CC 00

CO

a> o3

c*

1

2

e -

£ 1

OS

1-^^

IC

•r «> —

3

oo

CO C35 OS

o>

■Cil;

<

1

03 T? ^

03

o

Is

CM

CO

<35 r-H Tt<

CO 00 OS

s

w ® *"'

03

s

H

n

£ 1

■a

1«o|

l>.

CSOOO

0.-1 >o

^

^

03 03

.9 °

0)

©

1—1

£g 1

CO

O 3

o 1

■* t"- "5 l^

t^

OS

■* co -tf< CO

lO

cj ES

go

sT

INI

1^-° OS CO TT

CO

s

5/3

s

l|

*> 1

Ij

' I'd I

>»03

M W O.fcj

■g

O

CO

5

03 >>

S.ti

a+»

•2 cm

oo I lo

CO

50 1 1 «0

, — i

>>

C3

.2

s

'C

03

03

o

>

O i

O

Eh

to

03

H C8

§

O,

a

03

03

CM

O

O

'; i !

05

3

o o

OS

id

■* 00 CO

CC

o

oS

O —i
00 CO

CC
c-i

lO TfH r-i
-* Tfl 00

CM

8&

■coos

rH Tj- CM

s

CM lO

id in

■ -

■-H CM CD
|> CO rfi

" -^co

CO O

c

CO

CM r-l IQ

CO 05 CD
1

CO

CD«5
CO CM

CO CM-

>o

CC CO CO
COCO 00

T-H'eMrH

lO
CC

CMlO

CM CC H
CO OS CD

CM

COO
Or-J
CM CM

CM lO rH

•C 05 CD

r-l O 1-1

CM to

cot--:

©' cm' cm
»owc©

©00

•t' CM

00 OS

Nhn

C5 CO o"
O0 0OC5

CMO

00 00

CCCRO

t^t^oo

CO CM
1^00

!

CO CO OS
CO t^ t^

lO CO

r-l CO

>o

OXMOi

Hion

O
1C

00 00

II

oo

CM r-i

CO

■d

oo >d tj5

III

ass

OS

I'd

•- ca
f>HCC

CM

J s?

M O.CJ

jj-, ajQ

filCQE-1

CO

1 i

I HO I
1 1 CO 1

1 1 1^ 1

i

cj
o

-

o

Eh

03

o

Ih

Ph

>.

03

3
o

03 £0

"SSLa

P.t3 eg

m S C

o 3 c
PO'CU

16

CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

^
w =?

iS o

JO

» 1

—

=
a

oc Ci

. >-<

a

c
u

£' W

a

«

1

C33

K

•5 £

&3

eg

s
o

o -^

'A

<» «

e §

sS S

s e

HO

goo

a!*""1

-4-3
8

fcC

a

03

0)

O

fin a

Dollar

CC

©

m

c
c

00

1

8

!!!!!

1

|

3

o
Eh

? CMt-Os

gosCN OS

3

x

CO

CN

t^ iri iO iC

OOOSNiCN

o
t>:

CO

*iC*lON
CM 00 c: ■<*< i-l

tji" as' >d n id

cd

CM*

TJi

"3

1
O

£cCCMCO
g C2 t- CO

o

CO

X O "* iH
t- C5 lO »c

CO

c<5

O CM O IC 00
CM CO OS O O

X

id

Tt< CO i-t CM TJ<

ocxcx
cm" i-5 ' 1-5

CN

CO

cochin

"*

OO00N
CNCOi-lrt

3

© rh O «5 cc

•^ VC CO CO CO

"5

09

«i-iN^X
CO >C CM CO CN

s

CM

o
a

o
Q

X

8

00 00 00 Tf
lO00«O»Q

CO CM' CO CO

GO

CO

■* 00 Cn ic 00

OMi-iNX
■*' id Tfl CN CO

t-

d

CM

1 00 CN CN H CN
lOOIKNO

t-5 «j co ^' CM

t—

CN

a

*5 « .§ lO iO CN

o

CO

o
cr

o

o ic c o

n*HO
NiCNN

o

CM

to »o lO C IC

O itf* i-H CO •*

WOMCN

Q

oiooco

HMICOIO
»C H CO 00 LO

CO-
CO

03
o

03

o

o

-g cn i-5

00

as ic ■<*■ o

CO CC t- 00
CO' i-H i-i i-5

X
UO

00

■* 00 iji 00 ■«
CM 00 CO CO O

CO CM" <M' i-i CM

X
00

CN O — CM — i
CO t- t- O) TT

§

1 I Kl

sua

SJ CN CO CO

1

CO

1

CO lO 00 O
— < >o «o CO

%

ggg^gg

s

CO

i*ON^N
lO OS IQ N ■*

CM

CM

(-1

i 03

0 bo

CCCOCN

E-" i-i r-5 OS

t-

as i-5 « o

OOONTf

co ci d ■*' «

CO CN CM CN CM

HIOOOV
CN CNONCNCN

s

xi
p.

o

a

03^

>•-

£3T3

5 5

§ ^ —5 CO

C0I> ■* f

CO CM O CM
«o t- 0C 00

■<*• co co as ■^,

r-5 Tf5 O O CO
I> CC 00 t- 00

CO 00 CO CC •"*•

i*5 t-5 00 CN d
t- t- CO CC CC

SS

II

©c3

NWO

. cccn cc

&, 00 OS 00
o

EN

tN

X

co as oc cc

0C 1 1> t> t>.

;

Tf OS 1-1 CO H
r-' Tj5 e» — ' CO

00 00 !>• oot-

OS IQ OS OS CM

-INICNN
XNXXX

8

X

73
CO w

03

<

1

■^i-nooc
cc x os as

Nif it!CO
CC N 0O 00 O

CC 00 CO LO — ■
t- 00 00 OS CO

03
t-i

o

03

pq

CO •* i—i 1C

•O CO XOS

as t- tt< ir cc

Tjiccr-oc *

•O CO 0C CO IC
Tj<t- 00 00 OS

03 03

Eg

K °5 t> CM CN

g t CNCO^

si

-

X

ONOOi

CO
CM

CO C- CO Tji ■*
CO'* CO CM CO

CM

COCO CC CN CN
lO T CN CO CN

O
CO

.2 3

«.«

is

OSOCN

s» t-5 «c ■*'

STTT

su t- o o
0, . . .
""• OS t- M3

«i

-cHCMr-as

— ' as co' co

CM — " —

im

ui i-I as co"

CN CN -i i—

CM

cm as cot- cc

1 1 1 1 1

tj- CM OS COt-

t^ rf O 00' CO
ON CN CN — —

00
CN

^C0^t>-H

cm' as t> id re'

7TTT7

CC — i COtji t—

co cn as i>5 >d

CM CM — i rt i-

t-

CN

00

u

>> ca

c

«s i i i

►5 U C3?

CO

i-i

Its <A >

UO

8*

-2 ! !cc ! o !!co ' 'S ' '2

<sj i i OS i O ii i OC i iii i 00 i iii ICO i
j£ I IN i Tf< ii it- i i ' lO ' 1 1 I li-l

§ | ; j of ; j | ; j- ; ;'»-?

>

s
>

a
a

Si

O

"3
o

Eh

c.
c

-

>

CD
CO
O
«
03

D

5

o
Eh

S3
CO

O

O

3

3

"3

c
Eh

i

03

3

s

c
r-

ARTIFICIAL DRYING OF RICE ON THE FARM 17

Table 3 includes only those lots of rice on which data were ob-
tained for all drying operations. The average operating cost for
individual drying operations was approximately 0.6 cent per bushel
for the drier at DeWitt and approximately 0.8 cent per bushel for
the drier at Nome. Since four drying operations are usually re-
quired in drying rice harvested with a combined harvester-thresher,
the total operating cost might be expected to range between 2 and
4 cents per bushel exclusive of fixed charges. The difference in the
operating cost of the two plants was doubtless due in large measure
to the difference in size of the driers. When operated under similar
conditions, with rice of the same moisture content, the drier at
DeWitt would probably dry nearly three times as much rice in a
10-hour day as the one at Nome. Moreover, the power require-
ments shown for the one at DeWitt do not include that required in
cleaning and elevating the rice.

Tests made with the drier at DeWitt during the 1929 season show
an average cost for individual drying operations of approximately
0.45 cent per bushel exclusive of fixed charges. During that season,
however, a much larger quantity of rice was dried than in 1930
and the moisture content of the rice was higher when it was brought
to the drier. As a general rule rice with a high moisture content
dries at a more rapid rate than does rice with a low moisture con-
tent. For this reason the average cost per drying operation is less
when the initial moisture content of the rice is high; but the total
cost is more than when the initial moisture content of the rice is low.

The overhead expense, which involves interest on the investment
and depreciation, is a fixed annual cost, but when computed on a per-
bushel basis these fixed charges decrease as the total volume of rice
dried per season increases. Thus it is to the advantage of the owner
to dry a large quantity of rice each season in order to reduce the
overhead cost per bushel.

It is difficult to arrive at any definite value for interest on the
investment and depreciation of a rice drier. For instance, in one
case where observations were made the drier was installed adjacent
to a farm elevator in a locality in which the rice is handled in bulk,
whereas another was located in a warehouse in a section where rice
is handled in sacks. In the second instance it was necessary to
construct bins for storing the rice between drying operations, and
the elevating and conveying machinery was an added expense as it
was of no use except in connection with the drier.

In localities where rice is handled in bulk, storage bins are a part
of the regular equipment. Where rice is handled in sacks, bulk-
storage structures must be provided and the expense charged to the
drier. The drier at DeWitt represented an investment of approxi-
mately $4,550. Of this amount about $3,950 represented the cost of
the machinery and equipment installed and $600 the cost of the
building in which the drier was housed. The drier at Nome cost
approximately $2,000 installed, and the building, including storage
bins, about $9,000. Calculating 6 percent interest on the average in-
vestment, 10 percent depreciation on the machinery, and 5 percent
depreciation on the buildings, the annual fixed cost would be about
$561.50 for the drier at DeWitt and $980 for the one at Nome.

18

The length of life of a drier and auxiliary equipment depends to
some extent on the quantity of rice handled each season, but it is
doubtful whether variations in the annual usage of such equipment
under ordinary conditions would greatly influence the annual fixed
cost. The fixed cost per bushel, however, varies inversely with
the number of bushels dried per season. If the driers at DeWitt
and Nome each handled 10,000 bushels of rice per season, the fixed
cost would have been approximately 5.6 and 9.8 cents per bushel,
respectively, and the total cost of drying would have been approxi-
mately 8 and 13 cents per bushel, respectively. If 20,000 bushels
were handled per season, the annual fixed cost would be 2.8 and 4.9
cents per bushel, respectively, for the two plants.

EXPERIMENTAL DRIERS

The structure of the rice kernel is such that it may crack or rupture
because of changes in temperature and moisture content. As a gen-
eral rule, in the case of drying any product, the temperature of the
drying air is maintained as high as possible without injury to the
product, in order to save expense for power, labor, and fuel.

To determine the maximum drying-air temperature which could
be used without injury to the milling quality of the rice, tests were
conducted during the 1930 harvest season on an experimental drier
set up on a farm near DeWitt, Ark. This unit was furnished by a
grain-drier manufacturer and was of the same general design as a
commercial drier located on the farm where the tests were conducted.

The experimental drier was of the direct-heat type ; coke was used
for heating the drying air. The gases were drawn from the furnace,
mixed with outside air in a mixing chamber to produce the desired
temperature, and forced through the rice by a fan. The temperature
of the drying air was controlled by a thermostat which, by means of
air pressure, actuated a series of dampers regulating the quantity of
hot and cold air admitted. The rice passed down through the drier
by gravity and the rate of discharge was controlled by a swinging
damper. The drier was divided into two sections or compartments.
Hot air was supplied to the upper compartment for drying, and
ordinary air was used in the lower compartment for cooling the rice.

In 1931 a small laboratory drier was constructed at Nome, Tex.
This experimental drier was made up of a sheet-metal box about 18
by 20 inches in section by 30 inches in height and open at the top.
A screen bottom tray, about 8 inches in depth, was made to fit in the
top of this box. The bottom of the tray was divided into four
equal areas and a sheet metal cylinder, 4% inches in diameter and
8 inches in height, was supported in a vertical position in the center
of each area. The screen wire was cut out beneath each cylinder
and the edge of the wire was soldered to flanges at the bottom of the
cylinder. Four removable cylinders were made with screen-wire
bottoms and of suitable diameter to fit within the other four cylin-
ders which were in a fixed position.

The rice to be tested was placed in the removable cylinders and
the screen-bottom tray filled with rice to any desired depth to assist
in regulating the quantity of air forced through the test rice. At
the beginning of each test a quart sample of rice was placed in each

ARTIFICIAL DRYING OF RICE ON THE FARM 19

cylinder, filling it to a depth of about 4^ inches. A moisture test
was made on a smaller sample taken from the same lot of rice. The
quart sample of test rice was weighed before being placed in the
cylinder and at regular intervals during the drying process. By
knowing the moisture content and weight of the rice in the cylinders
at the beginning of the test, it was possible to compute the weight
of the rice when the moisture content had been reduced to 14 percent.

The air used for drying was heated by an electric heater and was
forced into the bottom of the box and up through the rice by a fan.
An anemometer was used in measuring the velocity of the air in
each cylinder after it had passed through the rice.

In all of the tests made with the experimental drier the effect of
drying on the rice kernel was checked against results obtained by
allowing rice from the same lot to dry naturally. A portion of each
sample had been placed in a wire basket in the laboratory where it
was allowed to remain until the moisture content had reached ap-
proximately 14 percent. No heated air was applied to these portions
of samples and no effort was made to force the evaporation of mois-
ture except that a fan was used to agitate the air slightly surround-
ing the baskets.

To ascertain by a definite test the effect of different methods of
drying on the milling quality of the rice, each sample was tested
for hardness with a Smith shelling device. This is the official device
used in testing samples for milling quality in applying the United
States standards (grades) for rough rice. The official conversion
table was used to convert the figures for shelling-device results into
terms of estimated yields of head rice per barrel.

TEMPERATURE OF DRYING AIR

In 1930 tests were made with the experimental drier at DeWitt
to determine the effect of the drying-air temperature on the milling
quality of rice. Most of these tests were made with the rice in con-
tinuous flow through the drier but some batch-drying tests were
made. Average results of all tests are shown in figure 9. The rice
dried with heated air showed a lower milling quality than samples
of the same lot of rice dried under ordinary air conditions. The
reduction in yield of head rice per barrel of rough rice was, how-
ever, greatly accelerated by the use of a drying-air temperature in
excess of approximately 120° F.

In 1931 a series of tests was conducted at Nome, in which samples
of individual lots of rice were dried to approximately 14 percent
moisture in one operation with the drying air at a temperature of
110°, 120°, 130°, and 140° F., and similar samples of the same lot
were dried at the same respective temperatures but exposed to the
drying air only 1 hour each day until dry.

Average results of such tests made with Early Prolific, Fortuna,
and Blue Rose rice are shown in figure 10.

Under the test conditions 120° F. seemed to be the maximum dry-
ing-air temperature that would not cause appreciable injury to the
rice when dried 1 hour each day. In practically all cases a lower
yield of head rice was indicated when samples of a given lot were
dried with heated air at any temperature than when samples were
allowed to dry naturally at ordinary air temperatures, but the yield

20

CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

decreased more rapidly with temperatures in excess of, than with
temperatures lower than, 120°.

The data also indicate that some varieties of rice are subject to
greater injury from high temperatures than others. The indications

Figure 9.
pounds
percent

110 120 130 140 150 160

Temperature of drying air {"£)

— Effect of the drying-air temperature on the estimated yield of head rice in
per barrel of rough rice when dried to a moisture content of approximately 14

from these tests are that Early Prolific rice will stand a higher
drying-air temperature than either Blue Rose or Fortuna. Samples
of Fortuna rice dried at one continuous operation at a temperature

100

V 70

?i!JLRose- 'n+ermittent

-&!&£!2!L«c-int(.rmiftent

/^Natural drying in laboratory

■"s^"5'

^c

^

\

.X

Experimental drier tests

^3?

110 120

Temperature of drying air (°F.)

130

Figure 10. — Effect of drying-air temperature and method of drying on the estimated yield
in pounds of head rice per barrel of rough rice when dried to a moisture content of
approximately 14 percent.

of 130° or 140° F. showed a higher milling quality than samples
of the same rice dried with an exposure of 1 hour each day at corre-
sponding temperatures. It may be that not a sufficient number of
tests were made with Fortuna rice to give reliable averages.

ARTIFICIAL DRYING OF RICE ON" THE FARM 21

AIR VELOCITY AND RATE OF DRYING

Observations and tests made with both commercial and experi-
mental driers prior to 1931 showed that rice may be seriously dam-
aged when high drying temperatures are used. It was not known,
however, whether the injury was caused by the temperature of the
drying air, the rate of drying, or the total quantity of moisture
removed at one drying operation. To obtain some information on
these points, samples of the same lot of Fortuna rice were dried
at different air velocities, but at the same temperature and with the
same period of exposure each day. The rice that was dried with a
velocity of approximately 88 feet per minute, or 88 cubic feet per
minute per square foot of area, with an exposure of 1 hour each
day until dry, showed a lower yield than that dried at a velocity of
45 feet per minute. A further reduction in yield was obtained with
an air velocity of about 130 feet per minute with the same tempera-
tures and period of exposure.

These results seem to indicate that the rate of drying and the total
quantity of moisture removed at one drying operation are chiefly
responsible for injury to the milling quality of the rice. This opinion
was also borne out in additional tests with Blue Rose rice, on which
a series of tests were made including variations in air velocity, tem-
peratures, and period of exposure for different samples of the same
lot. With one exception, a lowering of milling quality resulted with
an increase in air velocity, an increase in temperature, or an increase
in the period of exposure for each drying operation. An increase in
temperature, or an increase in air velocity, accelerated the rate of
evaporation of moisture, and this has a tendency to crack or rupture
the rice kernels. With either a high temperature or a high air ve-
locity, but with a short period of exposure, only a small quantity of
moisture was removed from the rice during each operation — prob-
ably the greater part of which was from the hulls and the outer sur-
face of the kernels — and little damage was done to the milling
quality of the rice. The effect of temperature on the rate of drying-
is indicated in figure 11. If either a high temperature or a high air
velocity is used, the time of exposure for each drying operation
should be reduced in order to obtain rice with the maximum yield
in pounds of head rice per barrel of rough rice.

It was also found that the time of exposure for each drying opera-
tion had a considerable influence on the rate of drying. When rice
is left in storage between drying operations the moisture tends to
become evenly distributed in the kernels. When it is again subjected
to the drying process the moisture in the hulls and in the outer coat-
ing of bran is given off quickly, but as diffusion of moisture from
within to the surface of the kernels begins, the rate of evaporation
is retarded. This is illustrated in figure 12 which shows that as the
period of exposure for each drying operation is decreased the total
time required to dry the rice is also decreased. Approximately 4
hours were required to reduce the moisture of rice from about 26 to
14 percent at a temperature of 120° F. when dried in one operation.
When samples of the same rice were dried with air under practically
the same conditions, but were exposed for an hour each day until dry,
the total time required was approximately 3 hours. With an expo-

22

CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

sure of 30 minutes each day, only 2 hours and 20 minutes were
required to dry the rice.

The retarding effect of diffusion of moisture to the surface of the
kernels on the rate of evaporation is illustrated in figure 12 by the

2 3

Time (hours)

Figure 11. — Effect of the temperature of the drying air at a velocity of approximately
47 feet per minute on the rate of drying Blue Rose rice when dried at one operation.

curve showing the rate of drying when the rice was exposed for
1 hour each day. For the first 30 minutes during the first drying
operation, the moisture content of the rice was reduced approxi-
mately 3.4 percent; during the remaining 30 minutes of the period

Figure 12. — Effect of time of exposure for each drying operation on the rate of drying
Blue Rose rice with air at a velocity of approximately 48 feet per minute and a tem-
perature of approximately 120° F.

the reduction in moisture was only 1.8 percent. For the first 30
minutes of the second drying operation the reduction was 2.8 percent
and for the second half of the operation only 1.1 percent. For the
first 30 minutes of the third drying operation there was a reduction

ARTIFICIAL DRYING OF RICE ON THE FARM

23

in moisture of 2.1 percent and for the remaining 25 minutes of the
test only 0.8 percent.

Figure 13 shows the effect of air velocity or quantity of air sup-
plied per minute per unit quantity of rice, on the rate of drying with
the air at a temperature of about 120° F. and with an exposure of 30
minutes each day. Results of these tests show that as the air velocity
is increased the rate of drying is also increased, but not in proportion
to the increase in velocity. This is because rice of a given moisture
content will lose moisture at a given rate provided the air conditions
are constant; but air velocity in excess of that necessary to remove
the moisture as it is lost by the rice kernel is of no additional value.

The air velocity, or quantity of air supplied per minute per bushel
of rice in a commercial drier, may vary depending in part upon the
make and model of drier used. In practically all commercial rice

Time (hours)

Figure 13. — Effect of air velocity at a temperature of approximately 120° F. on the rate
of drying Blue Rose rice with an exposure of 30 minutes daily.

driers the drying air is forced through thin layers or columns of rice,
for two reasons — to facilitate uniform drying and to permit the
circulation of a large volume of air through a given quantity of rice
without excessive power requirements.

With a given fan and within certain limits, the power required
varies with the cube of the quantity of air supplied when the fan
is operating against a given resistance. For this reason it would not
be advisable materially to increase the quantity of air supplied by a
given fan on a commercial drier without changing the design of the
drier. Even though a larger motor or engine might be installed to
supply the additional power required, the cost of power might off-
set any gain in rate of drying due to an increase in the quantity of
air supplied. According to tests made with both experimental and
commercial driers, air is usually supplied at the rate of from 100
to 150 cubic feet per minute per bushel of rice in the drier.

24 CIRCULAR 2 9 2, U.S. DEPARTMENT OF AGRICULTURE

EFFECT ON GERMINATION

Germination tests were made on samples of Blue Rose rice dried
in the laboratory under normal air conditions and in the experimental
drier at DeWitt with air at 110°, 120°, 130°, and 140° F. Results
of similar tests made on samples of Early Prolific, Fortuna, and
Blue Rose rice dried in the experimental drier at Nome, gave no indi-
cation of injury resulting from artificial drying at the temperatures
used. Additional tests made in a greenhouse on some of the samples
of rice gave no clear evidence of any difference in the seedlings from
the samples dried at different temperatures. Samples were obtained
from a lot of seed rice dried with a commercial drier with air at a
temperature of approximately 120° and no indication was found of
injury to germination resulting from the drying operation.

PRACTICAL CONCLUSIONS FOR THE TRADE

Artificially dried rice should remain in storage for a few days be-
fore it is milled or sampled for official grading.

Rice harvested with a combine should usually be cleaned before
drying.

If it is necessary to dry a given lot of rice in one operation, a
drying-air temperature in excess of 110° F. should not be used.

A drying-air temperature of 120° F. can be used without injury
to the rice if the moisture content is reduced only about 2 percent
at each drying operation and the rice is allowed to remain in storage
from 12 to 24 hours between dryings.

Small lots of rice should be consolidated whenever possible as bin
space, fuel, and labor can be conserved in drying large lots and in
keeping the drier in continuous operation.

Commercial driers will usually do the work for which they are
designed without changes or alterations on the part of the operator.
Under ordinary conditions no attempt should be made to increase the
quantity of air supplied by a fan as the increase in the power re-
quirements may offset any gain in rate of drying. In some driers,
however, the rice may funnel, causing uneven drying. This trouble
can usually be rectified on some driers by changing the position of
supporting members for the shut-off valves between the drier and
cooler and between the cooler and discharge hopper.

U.S. GOVERNMENT PRINTING OFFICE: 1933

For sale by the Superintendent of Documents, Washington. D.C. Price 5 cents